This application relates to optical sensing of various materials, including chemical and biological substances.
Nonlinear four wave mixing is an optical process in an optical medium where three coherent optical waves interact with one another through nonlinear coupling to produce a fourth coherent signal wave. The nonlinearities of the medium, primarily the third-order nonlinear susceptibility of the medium in some implementations, contribute to such nonlinear coupling. The signal wave includes information on optically-excited atoms or molecules present in the medium where the three input optical waves intersect and hence can be collected to extract information about the medium. The strength of the signal wave is associated with the population of atoms or molecules and the spectral characteristics of the signal wave can be analyzed to reveal the structure of the atoms or molecules of interest. The coherent characteristics of the four-wave mixing signal beam have a number of advantages, including a laser-like signal beam, efficient signal collection, excellent spatial resolution, and sub-Doppler spectral resolution. Hence, four-wave mixing has been widely used as a highly sensitive tool in spectroscopic measurements.
Various four wave mixing systems may be used for detection of a minute amount of a substance. For example, U.S. Pat. No. 5,600,444 to Tong describes devices and techniques for using two-input-beam forward-scattering degenerate four-wave mixing to achieve ultrasensative analytical measurements of an analyte. Backward-scattering degenerate four-wave mixing has also be used for sensitive laser spectroscopic detection. See, e.g., U.S. Pat. No. 6,141,094 to Tong.